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<head>
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    <style>
        .aa {
            border: 1px red solid;
            width: 200px;
        }
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</head>
<body>
<p class="text-muted">text-muted</p>
<p class="text-primary">text -primary</p>
<p class="text-success">text-success</p>
<p class="text-info">text-info</p>
<p class="text-warning">text-waring</p>
<p class="text-danger">text-danger</p>
<br>
<p class="bg-primary">bg-primary</p>
<p class="bg-success">bg-sucess</p>
<p class="bg-info">bg-info</p>
<p class="bg-warning">bg-waring</p>
<p class="bg-danger">bg-danger</p>
<br>
<button class="close">
    <span>  关闭按钮</span>
</button>
<span class="caret">三角符号</span><br>
<button class="btn btn-primary">选择课程 <span class="caret"></span></button>
<hr>
<br>
<div class="pull-left aa">
    快速向左浮动
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<div class="clearfix aa">清除浮动效果</div>
<div class="center-block aa">
    内容居中
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<div class="show aa">显示</div>
<div class="hidden aa">隐藏</div>
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